Darwin matters because evolution matters. Evolution matters because science matters. Science matters because it is the preeminent story of our age, an epic saga about who we are, where we came from, and where we are going.
The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. Their terrestrial origins are indicated by:
Their need to breathe air from the surface.
The bones of their fins, which resemble the limbs of land mammals.
The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.
The question of how a group of land mammals became adapted to aquatic life was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.
The molecular data is supported by the recent discovery of Pakicetus, the earliest proto-whale (see below). The skeletons of Pakicetus show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Proto-whales retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today’s artiodactyls and perissodactyls became herbivores later in their evolution. By contrast, whales retained their carnivorous diet, because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was not yet common. This may be why they were out-competed by better-adapted animals like the creodonts and later Carnivora which filled the gaps left by the dinosaurs.
Whales, like all mammals, evolved from reptiles, amphibians, and fish. Thus, over hundreds of millions they left the sea, grew legs, grew fur, and evolved lungs. Then they returned to the sea, lost their legs and fur, but kept their lungs.
A simply staggeringly complete fossil line..
More amazingly, Dr. Hans Thewissen, perhaps the leading specialist in whale evolution, tells us the transition from a completely land animal to a completely marine animal took only 8 million years.
On top of this, the end of whale evolution—the cetaceans, the order to which whales belong—are often more adept at life in the sea than fish! (Otters can outswim many fish, too!)
Cetaceans are mammals, the only mammals that live (all the time) in the water. The evolution of mammals will be covered elsewhere, so let’s skip that whole portion of the evolution of whales.
There are areas, from over 50 million years ago, where scientists are disagreed (and very excited about continuing to learn more) over which order whales evolved from. However, once we hit the pakicetids, 53 million years ago, the evidence starts nailing down the series for us.
Since we are excited to be able to go back 6 to 7 million years with man, the 53-million-year story of whale evolution is remarkably complete!
Summation of Whale Evolution
This series of fossils takes us from a hoofed land animal to a marine animal with back legs that cannot support it in just a few million years. The spine has changed, the limbs have shortened, the nostrils have moved, the skull has changed in shape, and the brain has changed in its function.
Along the way, all these species are linked by a special set of ear bones unique to this lineage. The timing of the fossils is right, and the progression is evident. It would be hard to imagine asking for better evidence of whale evolution.
This series of fossils is a powerful argument for evolution.